It depends on what features your web hosing company has available. There are some free e-commerce packages. If your host has the right capability, installing the software can be as simple as downloading a ZIP file, extracting it then uploading everything to a folder on the server by FTP, then creating a link to that folder on an HTML page on your site.
Another possibility is your host may already have one or more e-commerce packages you can directly install without downloading anything.
Of course after getting the software installed on the server, you have to configure its options and add your product information, photos etc.
One that's been around a while is Zen Cart. That is not one for the easily confused or folks with a low frustration threshhold! It's maintained by a community of programmers, shows it's a design by committee. Where *you* think some function should logically be is most likely where it will not be. To it's benefit, Zen Cart is used by a very large number of web stores and there's a very active forum. (With lots of "Where do I find...? type posts.) On the negative side most sites using Zen Cart tend to look very much alike due to the methods it uses to put pages together. It *can* be extensively customized to tone down the "Oh look, another Zen Cart site." style but it's not simple. It's based on OS Commerce and from what I've read it makes Zen Cart look easy.
I've been looking at PrestaCart. It's also free. They claim more than 50,000 web sites using it. The examples given have quite different styles. It's quite a bit newer than Zen Cart, but that means it has far less 'baggage' and 'legacy code'. It appears to have a core group working on it, which hopefully means they have a style guide so that instead of how Zen Cart uses on/off, enable/disable, true/false, or 1/0 in various places - all to do the same thing - they've chosen ONE pair of antonyms to enable or disable a function.
Both Zen Cart and PrestaCart have easy to setup connections to PayPal Premiere or Business accounts to accept credit cards.
If you plan on taking card payments for random or one-off items, Zen Cart has nothing built in to do that - unless you want to have to add the item with a quantity of 1, get paid for it, then go back in and delete it. If there's an addon to do it I haven't found it.
